SAR interferometry allows the derivation of phase fringes that can be related, for example, to the terrain topography, by exploiting the phases of focused images which are gathered in multiple satellite passes or in a single pass of a platform with two antennas (i.e. airborne interferometry).
Strip-mapping Synthetic Aperture Radar (SAR) is a microwave imaging system for generating high resolution terrain images from echoes collected by a relatively small antenna. Along-track (azimuth) resolution is achieved by combining many radar returns so that a large antenna is simulated. Across-track (slant range) resolution is achieved through time-delay measurements using time-dispersed linearly frequency modulated pulses that can be compressed into extremely short pulses. The practical limit to the azimuth resolution of present-day spaceborne SAR systems (i.e., SEASAT, ERS-1 JERS-1) is about 20-30 meters. This limitation exists principally in order to obtain a reasonable swath width and to avoid a huge amount of data to be transmitted to earth stations. Nonetheless, areas of interest can be easily imaged with higher azimuth resolution (1-2 meters) using the spot-light SAR technique, as disclosed by D.C. MUNSON Jr. et al. ("A Tomographic Formulation of Spot light August 1983 and by C. PRATI et al. ("Spot Mode SAR focusing with the .omega.-k Technique") in Proceedings of IGARSS'91 HELSINKI, June 1991.
On the other hand, the across-track resolution is practically limited to 7-8 meters (i.e. the transmitted signal bandwidth is limited to about 20 MHz) by power constraints.
A need has arisen for a SAR system having an increased across-track resolution of objects, for which two focused images are measured in the two receiving antennas separated slightly in the across-track direction or with only one antenna on two separate passes, this involving imaging on two separate orbits.
It has been shown by many authors that the largest baseline (i.e. the distance between the two takes in the across-track direction) that allows interferogram generation is limited by the ground range resolution, (i.e. the range bandwidth must exceed Nyquist rate for the interferometer fringes so that the speckle noise in the pixels being compared does not decorrelate).